The invention is based on a fuel injection pump for internal combustion engines including an "anti-erosion device".
In a known fuel injection pump of this type (U.S. Pat. No. 3,963,384). a thin wall pointing substantially toward a control edge serves as an anti-erosion device, dividing a diversion bore into two conduits over virtually its entire length by use of a partition wall. This partition wall is intended to prevent damage to the wall of the diversion bore, because the fuel stream that emerges upon diversion then essentially strikes only the partition wall and thus spares the bore wall. This known device for preventing erosion damage in the diversion bore is very expensive to manufacture, without even actually preventing damage to the bore wall. Depending on how this partition wall is disposed, there is always a problem that the end of the wall oriented toward the pump piston should not have any adverse effect on the diversion stream, which could go so far as to present the risk of throttling this stream by interfering with its flow energy. The wall end must also be prevented from coming into contact with the pump piston; and the wall must be secured against axial displacement in this direction. Naturally a means of securing the wall in the opposite direction must also be provided, which means that an expensive securing means is required, which is for example provided in the prior art pump housing, which in turn, because of the radial arrangement with respect to the pump cylinder, causes problems in installing the pump. It has also been correctly realized that as much as possible, this wall should have no contact at all with the wall of the diversion bore; however, this makes for still more problems in installation.
In another known fuel injection pump (European patent application No. 0114205), although the pump is not of the same generic type, it is also an object to prevent erosion in the diversion bore. A return flow conduit is provided between the chamber into which the diversion bore discharges and at a point in the diversion bore at which the least outflow pressure prevails upon diversion of the injection. This location of the mouth of the return flow conduit in the diversion bore at the point whee the lowest flow pressure, or the highest flow velocity, prevails is remote from the inlet of the diversion bore, and erosion never occurs except downstream of this point. As is well known, the cause of this erosion is considered to be cavitation, that is, destruction of material at the bore wall, which is caused by the implosion of hollow spaces in the fuel. These hollow spaces in the fuel are in turn generated by negative-pressure waves, which can be generated by the relief valve of the pump and by the negative wave throwback at the injection nozzle. Naturally these problems exist primarily in pumps with high pumping capacities and high pressures. The control edge is usually provided on an oblique control groove that is present in the pump piston and communicates hydraulically, for instance via a central bore, with the pump work chamber, which is disposed upstream from the end of the pump piston. Taking into account these physical characteristics of pumps, further disadantages of the above-mentioned generic type of pump become apparent. The flow pressure is not at all increased by providing a wall of this kind; quite the contrary, there is only a throttling effect inside the diversion bore, which can cause increased bubble formation and also lowers the operating speed of the pump, especially if this diversion bore also serves as an intake line during the intake phase. The backpressure also then building up to an increased extent may cause after-injections at the injection nozzle, which worsens the engine emissions.